A number of prior art processes have been proposed for the preparation of oxalate esters and carbonate esters by the oxidative carbonylation of alcohols with carbon monoxide. Such prior art processes encompass the use of expensive noble metal salt catalysts, oxygen, ferric or cupric salt redox systems and dehydrating agents.
The present invention is directed to an improved process for the oxidative carbonylation of alcohols wherein carbon monoxide and an alcohol are reacted at suitable temperatures and pressure conditions in the presence of specific metal salt catalysts and an unsubstituted or halogen substituted 2,5-cyclohexadiene-1,4-dione as the oxidant. The 1,4-benzene-diol or halogenated benzene diol formed as a result of the reaction can be reoxidized by known methods, e.g., with air or oxygen, to provide a cyclic process for the manufacture of the oxalate esters.
U.S. Pat. No. 3,393,136 describes a process for the preparation of oxalates by contacting carbon monoxide at superatmospheric pressure, with a saturated monohydric alcohol solution of a platinum group metal salt and a soluble ferric cupric salt (redox agent) while maintaining the salts in a highly oxidized state by the simultaneous introduction of oxygen or the application of a direct current electrical potential to the reaction zone. When oxygen is employed, explosive mixtures of oxygen and combustible organic vapors in the gas phase must be avoided and water scavengers or dehydrating agents such as alkyl orthoformic acid esters must be added to the liquid phase to prevent the accumulation of water.
A recent West German Pat. No. 2,213,435 discloses a method for the synthesis of oxalic acid and oxalate esters in water and alcohol respectively. A platinum group metal salt, a salt of a metal more electropositive than the platinum group metal, e.g. copper (II) chloride and an alkali metal salt comprise the catalyst. Oxygen in stoichiometric amounts was employed as the oxidant. A disadvantage of such reaction is that explosive mixtures of oxygen and carbon monoxide are necessary to effect reaction. Under non-explosive conditions only trace amounts of oxalate can be obtained.
In a recent article by Donald M. Fenton and Paul J. Steinwand, Journal of Organic Chemistry, Vol. 39, No. 5, 1974, pp. 701-704 the synthesis of oxalate and carbonate esters is described by reacting carbon monoxide and an alcohol in the presence of palladium (II) chloride redox systems, oxygen and dehydrating agents. The use of benzoquinone as an oxidant and palladium (II) chloride catalyst with or without a dehydrating agent is also shown. In the absence of a dehydrating agent and at low CO pressure the yields of oxalate and carbonate were 2.4 and 2.8 mole percent respectively based on the benzoquinone charged.
The oxalate products of the present invention have many important commercial applications, for example, as cellulose ether or ester resin solvents, for the preparation of pharmaceuticals and glycols and as dye intermediates.
Advantages of the present invention, as compared to prior art processes are (1) high conversions and high selectivity to oxalate esters over the attendant carbonate ester, (2) catalysis of the oxidative carbonylation reaction with less expensive and more readily available metal salt catalysts such as copper (II) chloride and zinc chloride, (3) elimination of hazardous operational conditions by avoiding explosive mixtures of oxygen and carbon monoxide and (4) avoiding the necessity of employing dehydrating agents to remove water.